(a) Field of the Invention
The present invention relates to an optical recording and read-out lens system to be used for recording signals onto optical recording type disks such as digitally recorded audio disks, video disks, etc. and for reading out signals recorded on optically recorded disks.
(b) Description of the Prior Art
As a recording and read-out method for optical recording type disks, there is such method to use two different wavelengths of light, one wavelength of light being used for control such as tracking and focusing and for reading out of signals and the other wavelength of light being used for recording of signals.
In this method, such operations are performed that the recorded signals are read out just after recording, it is judges whether they are correct or not, and correction is made when an incorrect signal is found. Therefore, for the lens system to be used in this method, chromatic aberration should be corrected so that the focal length becomes the same for both of the two different wavelengths of light to be used.
Besides, to increase the energy efficiency at the time of recording and to form a sharp pit, it is desirable to make the size of light spot on the disk small. For this purpose, the lens system sould have N.A. of 0.5 or more and its aberrations should be corrected very favourably.
Moreover, the disk rotating at high speed generally causes vertical vibration, horizontal vibration, irregular rotation, etc. To follow up these high speed movements, focusing and tracking are carried out by moving the lens system and, in some driving methods, correction of irregular rotation is also carried out by moving the lens system. Therefore, the optical recording type disks should be small in size and light in weight.
Moreover, to prevent the optical recording and read-out lens system from contacting the disk even when misoperation occurs, the working distance of the lens system should be made long. To reduce the loss in intensity of light to be caused by the optical recording and read-out lens system, the number of lenses constituting the lens system should be made small and, at the same time, surfaces of lenses which are in contact with air should be provided with antireflection coatings which are effective over the range of wavelengths of light to be used.
However, among known lens systems for optically recorded disks, there is almost no lens system which fulfills all of the above-mentioned requirements. For example, in cases of lens systems disclosed in Japanese published unexamined patent application Nos. 124660/75 and 66843/76, correction of chromatic aberration is not considered especially and, therefore, chromatic aberration is not corrected satisfactorily favourably.
The lens system disclosed in Japanese published unexamined patent application No. 133046/76 is designed for the purpose of the use with the light of short wavelength. Therefore, all lenses constituting the lens system are arranged separately from each other in order to prevent absorption of light by a cemented surface and, consequently, the lens system is large in size.
In cases of lens system disclosed in Japanese published unexamined patent application No. 18557/76 and lens system disclosed in Japanese published unexamined patent application No. 4068/80, which has lens configuration as shown in FIG. 1, a cemented doublet is used in order to eliminate chromatic aberration. However, the main purpose of using the cemented doublet is to make the lens system small in size and light in weight. Therefore, chromatic aberration is not eliminated satisfactorily.
Besides, in cases of the above-mentioned known lens systems, the working distance is short except the lens system according to Japanese published unexamined patent application No. 4068/80.
Among the known lens systems mentioned so far, there is almost no lens system which fulfills all of the requirements for an optical recording and read-out lens system for optical recording type disk. Especially, chromatic aberration is favourably corrected for very few of those lens system.